Tumor markers are synthesized by malignant cells and released into the bloodstream. Such markers may also be produced by host tissues in response to invasion or as a result of tumor-induced metabolic changes. Tumor marker levels in blood or tissue fluids are helpful in diagnosing, screening, and monitoring tumor progression or regression. An ideal tumor marker would allow a simple blood test to detect cancer, and its levels would correlate with the stage of tumor progression. Due to the lack of sensitivity and specificity, however, no single marker has been previously established in a general healthy population or in most high-risk populations. The use of tumor markers in cancer diagnostics is well described (Sluss, P. M. et al [2004] “ESTABLISHMENT OF A CENTRAL LABORATORY SERUM TUMOR MARKER SERVICE ON A CONSOLIDATED IMMUNODIAGNOSTIC PLATFORM: DEVELOPMENT OF PRACTICE STANDARDS, SERVICE IMPROVEMENTS, AND OPERATIONAL EFFICIENCY,” Clin Leadersh Manag Rev. 18 [1]:25-31; Gion, M. [2000] “SERUM TUMOUR MARKERS: FROM QUALITY CONTROL TO TOTAL QUALITY MANAGEMENT,” Breast 9[6]:306-11; Wiesner, A. [2004] “DETECTION OF TUMOR MARKERS WITH PROTEINCHIP® TECHNOLOGY,” Curr Pharm Biotechnol. February; 5[1]:45-67; Crawford, N. P. et al. [2003] “TUMOR MARKERS AND COLORECTAL CANCER: UTILITY IN MANAGEMENT,” J Surg Oncol. 84[4]:239-48; Agnantis, N. J. et al [2003] “TUMOR MARKERS. AN UPDATE APPROACH FOR THEIR PROGNOSTIC SIGNIFICANCE. PART I. IN VIVO,” 17[6]:609-18; Riley, R. D. et al. [2004] “A SYSTEMATIC REVIEW OF MOLECULAR AND BIOLOGICAL TUMOR MARKERS IN NEUROBLASTOMA,” Clin Cancer Res. 10[1 Pt 1]:4-12; Given, M. et al. [2000] “THE PREDICTIVE OF TUMOUR MARKERS CA 15-3, TPS AND CEA IN BREAST CANCER RECURRENCE,” Breast. 9[5]:277-80).
Currently available cancer markers measure cancer antigens. For example, prostate cancer can be diagnosed by measuring the prostate-specific antigen (PSA) cancer marker (Gretzer, M. B. et al. [2003] “PSA MARKERS IN PROSTATE CANCER DETECTION,” Urol Clin North Am. 30[4]:677-86). The Carcino-Embryonic Antigen (CEA) marker has been found to have diagnostic utility in assessing colorectal cancer (Crawford, N. P. et al. [2003] “TUMOR MARKERS AND COLORECTAL CANCER: UTILITY IN MANAGEMENT,” J Surg Oncol. 84[4]:239-48). The cancer antigen, CA15-3, has been correlated with breast cancer (Cheung, K. L. et al. [2003] “OBJECTIVE MEASUREMENT OF REMISSION AND PROGRESSION IN METASTATIC BREAST CANCER BY THE USE OF SERUM TUMOUR MARKERS,” Minerva Chir. June; 58[3]:297-303). The cancer antigen, CA19-9, has been employed to diagnose gastrointestinal cancer (Grotowski, M. [2002] “ANTIGENS [CEA AND CA 19-9] IN DIAGNOSIS AND PROGNOSIS COLORECTAL CANCER,” Pol Merkuriusz Lek. 12[67]:77-80; Trompetas, V. et al. [2002] “GIANT BENIGN TRUE CYST OF THE SPLEEN WITH HIGH SERUM LEVEL OF CA 19-9,” Eur J Gastroenterol Hepatol. 14[1]:85-8). The cancer antigen, CA125, has been used to diagnose ovarian cancer (Anderiesz, C. et al [2003] “SCREENING FOR OVARIAN CANCER,” Med J Aust. 178[12]:655-6).
The majority of solid tumors show chromosomal instability caused by aberrations in chromosomal segregation during cell division. Several enzymatic kinases are involved in maintaining proper chromosomal segregation and regulating cell cycle progression. One such kinase, cAMP-dependent protein kinase (PKA), appears to have a functional role in many aspects of cell signaling, metabolism, and proliferation (Matyakhina, L. et al. [2002] “PROTEIN KINASE A AND CHROMOSOMAL STABILITY,” Ann NY Acad Sci. 968:148-57; Tortora, G. et al. [2002] “PROTEIN KINASE A AS TARGET FOR NOVEL INTEGRATED STRATEGIES OF CANCER THERAPY,” Ann NY Acad Sci. 968:139-47).
Mammalian cells possess two types of cAMP-dependent protein kinase (PKA) species (Krebs, E. G. et al [1979] “PHOSPHORYLATION-DEPHOSPHORYLATION OF ENZYMES,” Annu Rev Biochem. 48:923-39). These protein kinases are designated type I (PKA-I) and type II (PKA-II); they are distinguished by different regulatory subunits (R subunits) RI and RII, and share a common catalytic subunit (C subunit) (Beebe, S. J. et al. [1986] “CYCLIC NUCLEOTIDE-DEPENDENT PROTEIN KINASES,” In: The Enzymes: Control by Phosphorylation, E. G. Krebs et al. [Eds] Academic Press: Orlando and London. pp. 43-11).
Traditionally, the enzyme activity of protein kinases has been assayed by following the transfer of a radioactive phosphate group from (γ-32P) ATP to a residue of a suitable protein or peptide substrate (See, e.g., Witt, J. J. et al. [1975] Anal Biochem. 66:253-8; Casnellie, J. E. [1991] Methods Enzymol. 200:115-20; U.S. Pat. No. 6,498,005). PKA enzyme assays have been described (Cohen, C. B. et al. [1999] “A MICROCHIP-BASED ENZYME ASSAY FOR PROTEIN KINASE A,” Anal Chem. [1999] 273:89-97; Cho, Y. S. et al [2000] “EXTRACELLULAR PROTEIN KINASE A AS A CANCER BIOMARKER: ITS EXPRESSION BY TUMOR CELLS AND REVERSAL BY A MYRISTATE-LACKING CALPHA AND RIIBETA SUBUNIT OVEREXPRESSION,” Proc Natl Acad Sci USA. 97[2]:835-40).
Through biochemical studies and gene cloning, four isoforms of the R subunits, RIα, RIβ, RIIα, and RIIβ, have been identified (Amieux, P. S. et al [2002] “THE ESSENTIAL ROLE OF RI ALPHA IN THE MAINTENANCE OF REGULATED PKA ACTIVITY,” Ann NY Acad Sci. 968:75-95; McKnight, G. S. et al. [1988] “ANALYSIS OF cAMP-DEPENDENT PROTEIN KINASE SYSTEM USING MOLECULAR GENETIC APPROACHES,” Recent Prog Honn Res. 44:307-35; Levy, F. O. et al. [1988] “MOLECULAR CLONING, COMPLEMENTARY DEOXYRIBONUCLEIC ACID STRUCTURE AND PREDICTED FULL-LENGTH AMINO ACID SEQUENCE OF THE HORMONE-INDUCIBLE REGULATORY SUBUNIT OF 3′,5′-CYCLIC ADENOSINE MONOPHOSPHATE-DEPENDENT PROTEIN KINASE FROM HUMAN TESTIS,” Mol Endocrinol. 2:1364-73).
Importantly, the ratios of PKA-I to PKA-II can change dramatically during cell development, differentiation, and transformation (Lohmann, S. M. et al. [1984] “REGULATION OF THE CELLULAR AND SUBCELLULAR CONCENTRATIONS AND DISTRIBUTION OF CYCLIC NUCLEOTIDE-DEPENDENT PROTEIN KINASES,” In: Advances in Cyclic Nucleotide and Protein Phosphorylation Research, P. Greengard et al. [Eds] Raven Press: New York. pp. 63-117; Cho-Chung, Y. S. [1990] “ROLE OF CYCLIC AMP RECEPTOR PROTEINS IN GROWTH, DIFFERENTIATION, AND SUPPRESSION OF MALIGNANCY: NEW APPROACHES TO THERAPY,” Cancer Res. 50:7093-100; Cho-Chung, Y. S. [2003] “cAMP SIGNALING IN CANCER GENESIS AND TREATMENT,” Cancer Treat Res. 115:123-43).
The cAMP signaling pathway has been proposed as a therapeutic target in lymphoid malignancies (Lerner, A. et al. [2000] “THE cAMP SIGNALING PATHWAY AS A THERAPEUTIC TARGET IN LYMPHOID MALIGNANCIES,” Leuk Lymphoma. 37[1-2]:39-51; Cho-Chung, Y. S. et al. [1995] “cAMP-DEPENDENT PROTEIN KINASE: ROLE IN NORMAL AND MALIGNANT GROWTH,” Crit Rev Oncol Hematol. 21[1-3]:33-61; Cho-Chung, Y. S. et a. [1993] “THE REGULATORY SUBUNIT OF cAMP-DEPENDENT PROTEIN KINASE AS A TARGET FOR CHEMOTHERAPY OF CANCER AND OTHER CELLULAR DYSFUNCTIONAL-RELATED DISEASES,” Pharmacol Ther. 60[2]:265-88). Increased expression of RIα/PKA-I has been shown in human cancer cell lines and primary tumors, as compared with normal counterparts (Cho-Chung, Y. S. [1990] “ROLE OF CYCLIC AMP RECEPTOR PROTEINS IN GROWTH, DIFFERENTIATION, AND SUPPRESSION OF MALIGNANCY: NEW APPROACHES TO THERAPY,” Cancer Res. 50:7093-100; Miller, W. R. et al [1993] “TYPES OF CYCLIC AMP BINDING PROTEINS IN HUMAN BREAST CANCERS,” Eur J Cancer. 29A:989-91) in cells after transformation with chemical or viral carcinogens and the Ki-ras oncogene or transforming growth factor-α, and on stimulation of cell growth with the granulocyte-macrophage colony-stimulating factor or phorbol esters (Cho-Chung, Y. S. [1990] “ROLE OF CYCLIC AMP RECEPTOR PROTEINS IN GROWTH, DIFFERENTIATION, AND SUPPRESSION OF MALIGNANCY: NEW APPROACHES TO THERAPY,” Cancer Res. 50:7093-100; Cho-Chung, Y. S. et al. [2002] “DISSECTING THE CIRCUITRY OF PROTEIN KINASE A AND cAMP SIGNALING IN CANCER GENESIS: ANTISENSE, MICROARRAY, GENE OVEREXPRESSION, AND TRANSCRIPTION FACTOR DECOY,” Ann NY Acad Sci. 968:22-36). Conversely, a decrease in the expression of RIα/PKA-I correlates with growth inhibition induced by site-selective cAMP analogs and antisense oligonucleotides targeted against the RIα subunit of PKA in a broad spectrum of human cancer cell lines and human tumors grown in nude mice (Cho-Chung, Y. S. et al. [1989] “SITE-SELECTIVE CYCLIC AMP ANALOGS AS NEW BIOLOGICAL TOOLS IN GROWTH CONTROL, DIFFERENTIATION AND PROTO-ONCOGENE REGULATION,” Cancer Inv. 7:161-77; Cho-Chung, Y. S. et al [1999] “ANTISENSE DNA-TARGETING PROTEIN KINASE A-RIα SUBUNIT: A NOVEL APPROACH TO CANCER TREATMENT,” Front Biosci. 4:D898-D907).
It has been previously demonstrated that various cancer cell types excrete PKA into the conditioned medium (Cho, Y. S. et al. [2000] “EXTRACELLULAR PROTEIN KINASE A AS A CANCER BIOMARKER: ITS EXPRESSION BY TUMOR CELLS AND REVERSAL BY A MYRISTATE-LACKING CALPHA AND RIIBETA SUBUNIT OVEREXPRESSION,” Proc Natl Acad Sci USA. 97[2]:835-40). This extracellular protein kinase A (ECPKA) is present in active, free catalytic subunit (C subunit) form (“PKA Cα”) and its activity is specifically inhibited by the PKA inhibitory protein PKI. Overexpression of the Cα or RIα subunit gene of PKA in an expression vector, which upregulates intracellular PKA-I, markedly upregulates ECPKA expression. In contrast, overexpression of the RII subunit—which eliminates PKA-I, upregulates PKA-II, and reverts the transformed phenotype—downregulates ECPKA. A mutation in the Cα gene that prevents myristylation allows intracellular PKA upregulation but blocks the ECPKA increase, suggesting that the NH2-terminal myristyl group of Cα is required for ECPKA expression. In the serum of cancer patients, ECPKA expression is markedly upregulated, in contrast to normal serum (Cho, Y. S. et al. [2000]“EXTRACELLULAR PROTEIN KINASE A AS A CANCER BIOMARKER: ITS EXPRESSION BY TUMOR CELLS AND REVERSAL BY A MYRISTATE-LACKING CALPHA AND RIIBETA SUBUNIT OVEREXPRESSION,” Proc Natl Acad Sci USA. 97[2]:835-40).
The development of monoclonal antibodies has led to the identification of numerous tumor-associated antigens in the serum and tissues of patients with malignancies. Protein products of oncogenes and tumor suppressor genes can be detected in extracellular fluids and serve as potential markers for carcinogenesis in vivo. Some of these growth factors are encoded by oncogenes. For example, higher levels of p21-ras protein are encoded by the ras oncogenes found in patients' blood. Circulating antibodies against p53 tumor suppressor protein have been found in sera of patients with breast and lung carcinomas and in children with B-lymphomas (Winter, S. F. et al. [1992] “DEVELOPMENT OF ANTIBODIES AGAINST P53 IN LUNG CANCER PATIENTS APPEARS TO BE DEPENDENT ON THE TYPE OF P53 MUTATION.” Cancer Res. 52:4168-74; Lubin, R., et al. [1993] “ANALYSIS OF P53ANTIBODIES IN PATIENTS WITH VARIOUS CANCERS DEFINE B-CELL EPITOPES OF HUMAN P53: DISTRIBUTION ON PRIMARY STRUCTURE AND EXPOSURE ON PROTEIN SURFACE.” Cancer Res. 53:5872-6; Crawford, L. V., et al. [1982] “DETECTION OF ANTIBODIES AGAINST THE CELLULAR PROTEIN P53 IN SERA FROM PATIENTS WITH BREAST CANCER.” Int. J. Cancer. 30:403-8). Antibodies against oncogenes such as c-myc and c-myb have also been found in sera of patients with colorectal and breast tumors (Sorokine, I., K. et al. [1991] “PRESENCE OF CIRCULATING ANTI-C-MYB ONCOGENE PRODUCT ANTIBODIES IN HUMAN SERA.” Int. J. Cancer. 47:665-9; Ben-Mahrez, K., et al. [1988] “DETECTION OF CIRCULATING ANTIBODIES AGAINST C-MYC PROTEIN IN CANCER PATIENT SERA.” Br J Cancer. 57:529-34; Ben-Mahrez, K., et al. [1990] “CIRCULATING ANTIBODIES AGAINST C-MYC ONCOGENE PRODUCT IN SERA OF COLORECTAL CANCER PATIENTS.” Int J Cancer. 46:35-8).
In addition to the above-mentioned novel markers, some other proteins, hormones, and enzymes have been used as markers for the past 30 years. Notable among these are carcinoembryonic antigen (CEA), α-fetoprotein (AFP), human chorionic gonadotropin (HCG), and prostatic acid phosphatase (PAP). Most of these markers lack specificity, however. These levels are also increased under benign conditions and during gestation. All of these markers are based on the antigen determination method; the markers are lack of specificity and sensitivity. There is great need to discover novel biomarkers and translate them to routine clinical use. The present invention is directed to such need.